Brush seals are used to minimize leakage through a gap between two components, wherein such leakage is from a higher pressure area to a lower pressure area. The two components may both be rotating, may both be stationary, or one may be rotating and the other stationary. Brush seals may have any shape including, but not limited to, annular or straight. Brush seals have been used, or their use proposed, in rotating machinery such as, but not limited to, turbomachinery including steam turbines and gas turbines used for power generation and gas turbines used for aircraft and marine propulsion. It is noted that brush seals prevent the leakage of steam in steam turbines and prevent the leakage of compressed air or combustion gases in gas turbines.
A steam turbine has a steam path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, gas-path leakage in the turbine area of a gas turbine (such as between the rotating tips of turbine rotor blades and the circumferentially surrounding turbine casing) will lower the efficiency of the gas turbine leading to increased fuel costs. Also, gas-path leakage in the combustor area of a gas turbine will require an increase in burn temperature to maintain power level, such increased burn temperature leading to increased pollution, such as increased NOx and CO production.
Gas-path leakage occurs through gaps between larger gas turbine components such as through gaps between the combustor and the turbine, and gas-path leakage occurs through gaps between smaller gas turbine components such as through gaps between combustor casing segments. Such components have surfaces of different shapes, suffer from assembly misalignment, and undergo vibration. For example, vibration is particularly important during startup of a turbine rotor which must pass through one or more critical frequencies before reaching operational speed. Also, hot-section components, such as rotors and circumferentially surrounding stators, thermally experience hot gas flow and typically undergo different thermal growths. Even rotor blades (also called buckets) in the same circumferential row experience different thermal or centrifugal growths leading to "bumps" (i.e., different radial clearance gaps). Steam-path leakage occurs through gaps between steam turbine components in a manner similar to that for gas-path leakage through gaps between gas turbine components.
Conventional brush seal designs have been proposed for use in gas-path leakage gaps of gas turbines and for use in steam-path leakage gaps of steam turbines. Such brush seals have wire or ceramic bristles conventionally welded or otherwise affixed to a backing plate. Typically, such brush seals align their wire bristles to contact a rotating shaft at an angle between generally forty-five and generally sixty degrees with respect to a radius line from the shaft to the point of bristle contact, such bristle alignment creating a softer, longer-wearing brush-seal contact.
The problem of brush seal hysteresis is known in the art. Gas or steam from the higher pressure area will set the bristles against the backing plate during periods when the gap between the two components is smaller (due to differential thermal growth, vibration, or "bumps") and the bristle ends touch, and are deflected by, one of the components. Such bristles will stay set and such bristle ends will be spaced further apart from the one component when the gap returns to normal size. Such bristle set (i.e., hysteresis) causes greater leakage and renders the brush seal largely ineffective. Equalizing the pressure on both sides of the gap will release the bristles. A known technique is to direct some higher pressure gas or steam from the front to behind the bristles to prevent the bristles from being set against the backing plate. However, such gas or steam bypass is itself an undesirable gas or steam leakage.
What is needed is an improved brush seal which prevents hysteresis without directing any higher pressure gas or steam from the front to behind the bristles.